The present invention relates to the field of mobile communication between a base station and a mobile terminal, and in particular to connections with high transmission rates in a Time Division Multiplex Access (TDMA) system, such as the GSM system.
In a TDMA system, communication between the base station and a mobile terminal takes place in channels. A number of channels are transmitted on one carrier frequency using time division multiplex. The transmission on each carrier frequency takes place in time slots, and each physical channel occupies one time slot. As an example, in GSM, eight physical channels at a time may share the same carrier frequency, that is, eight time slots constitute one frame. One traffic channel (TCH) occupies one physical channel, and one connection usually involves one TCH. In this way, all connections are transferred at the same maximum bit rate. At present, the maximum bit rate is 14.4 kbit/s for payload information.
According to the GSM standard, frequency hopping may be used, that is, at regular intervals the carrier frequency is changed, to minimize the effects of multipath fading and the disturbances between the channels.
The standards High Speed Circuit Switched Data (HSCSD) and Global Packet Radio Services (GPRS) have been introduced in the GSM standard to enable connections with higher transmission rates. HSCSD and GPRS connections use a multislot configuration of channels for transmitting data, that is, one connection is allowed to occupy more than one channel, that is, more than one time slot in each flame. The network architecture to support HSCSD allows a maximum of eight independent full rate traffic channels to be used for one connection, to achieve a bit rate eight times higher than the normal bit rate.
Different mobile terminals are capable of handling different numbers of channels. The maximum number of uplink channels and downlink channels, respectively, and the total maximum number of channels may be restricted. In the GSM standard, 18 different mobile classes have been defined, specifying the number of channels that a mobile terminal is able to handle. Other restrictions are also specified; for example some mobile terminals can only handle consecutive channels in a multislot connection, whereas others can handle any combination of channels. The simplest mobile class only handles one uplink channel and one downlink channel. The most advanced mobile class handles up to eight channels in each direction, and any combination of channels may be used.
The GSM standard states that all channels to be allocated to one mobile connection in a multislot configuration must have the same Training Sequence Code (TSC), Hopping Sequence Number (HSN), Mobile Allocation (MA) and Mobile Allocation Index Offset (MAIO), if frequency hopping is used. If frequency hopping is not used, all channels used in the same multislot configuration must have the same TSC and Absolute Radio Frequency Channel (ARFCN). This means that all channels in a multislot configuration transmit and receive on the same frequencies in the same frame, with the same TSC, even when frequency hopping is used.
If the desired number of channels cannot be allocated to a connection, two different results are possible. Normally, transparent connections are refused, whereas non-transparent connections, for which there is no strict requirement for a certain data rate, are set up with the highest possible bandwidth. If a physical channel that may be used with the multislot connection becomes free, this physical channel may then be added to the multislot connection.
In most mobile telephony systems, channel spacing is used, that is, the uplink carrier frequency and the downlink frequency used in a connection are always at a specified distance from each other. When a carrier frequency has been selected for one direction, the carrier frequency to be used in the other direction is given. Thus, when high transmission rates are desired in both directions, it must be ensured that the desired number of channels are available on both the uplink and the downlink carrier frequencies.
If no such channels become free, the bandwidth of the connection will not be increased. Also, if a channel is released it may be occupied again by another connection before it is discovered and included in the multislot connection. The present solutions thus do not utilize the network resources in an optimal way. Allocation of channels may fail because bandwidth that is available may not be utilized.
Co-pending Swedish patent application 9703538-0 discloses a method of allocating channels to multislot connections in a mobile telecommunication network, in which all available channels into Multislot Resource Groups, each Multislot Resource Group comprising all channels that may be assigned to one multislot connection. When a multislot connection is to be established, the most suitable resource group is selected, as the resource group having a sufficient number of idle channels, but as few idle channels as possible above this number, and fulfilling a number of other requirements such as interference requirements.
This document only describes low to optimize the allocation of available resources in a current network situation. No solution is offered in the case when the requested number of channels cannot be allocated.
It is an object of the present invention to increase the probability of achieving multislot connections having a desired bandwidth.
It is another object of the invention to increase the chance of utilizing neighbouring channels that are occupied when a multislot connection is set up and released during the connection.
It is yet another object of the invention to establish multislot connections having the desired bandwidth faster than what is possible with prior art methods.
It is another object to optimize the utilization of the capacity in the network.
These objects are achieved according to the present invention by a method in a mobile communication network of increasing the chance of achieving multichannel connections, comprising the following steps:
determining if there is a need to make certain physical channels idle to facilitate multichannel connections,
marking the channel or channels that should be made idle.
The marked channels may be moved actively, in which case the method also comprises the following steps:
selecting the channels to which the connections on said marked channels should be moved,
initiating the moving of said connections to said selected channels.
The marking and/or moving of the connection or connections is initiated, for example, when a predetermined number of attempts to allocate multichannel connections have failed, or when the network situation is such that the chance of establishing multichannels is low and should be increased. It may also be initiated when an establishment of a multichannel connection is attempted unsuccessfully, in which case the following steps are performed:
putting the multichannel connection on hold,
moving the connections on the marked channels.
setting up the multichannel connection when sufficient bandwidth is available.
According to a preferred embodiment, the connections are moved using the intra-cell handover procedure.
If channels cannot be moved, the following steps may be performed:
setting up the multichannel connection with a lower bandwitdh than requested,
waiting until the marked channels become idle,
including the marked channels in the multichannel connection.
A control unit in a mobile communications network is also described, said unit comprising:
means for determining if there is a need to make certain physical channels idle to facilitate multichannel connections,
means for marking the channel or channels that should be made idle.
The control unit may also comprise means for selecting a channel or channels to which connections may be moved and means for initiating the moving of a connection on a marked physical channel to a selected physical channel.
The invention offers the following advantages:
The probability of success in achieving high transmission rates for multislot channel allocations is increased compared to the prior art solutions.
It allows a more effective use of the capacity when multislot connections are used.
According to the invention, only the desired bandwidth of the connection is taken into account. Generally a number of parameters have to be considered when setting up a connection, including the properties of the mobile terminal. The above mentioned co-pending Swedish patent application 9703538-0 discloses a method for allocating multislot channels according to several of these parameters.